1. Field of the Invention
The present invention relates to a photoelectric conversion device which has a non-single-crystal semiconductor laminate member having formed therein at least one PIN junction, and a method for manufacture of such a photoelectric conversion device.
2. Description of the Prior Art
A photoelectric conversion device of the type including a non-single-crystal semiconductor laminate member having formed therein at least one PIN junction usually has the non-single-crystal semiconductor laminate member formed on a substrate having a conductive surface and a conductive layer formed on the non-single-crystal semiconductor laminate member. The non-single-crystal semiconductor laminate member has at least a first non-single-crystal semiconductor layer of a P or N first conductivity type, an I-type second non-single-crystal semiconductor layer formed on the first non-single-crystal semiconductor layer and a third non-single-crystal semiconductor layer formed on the second non-single-crystal semiconductor layer and having a second conductivity type opposite from the first conductivity type. The first, second and third non-single-crystal semiconductor layers form one PIN junction.
In this case, for example, the substrate has such a structure that a light-transparent conductive layer is formed as a first conductive layer on a light-transparent insulating substrate body. The first and third non-single-crystal semiconductor layers of the non-single-crystal semiconductor laminate member are P- and N-type, respectively. Further, the conductive layer on the non-single-crystal semiconductor laminate member is formed as a second conductive layer on the N-type third non-single-crystal semiconductor layer.
With the photoelectric conversion device of such a structure as described above, when light is incident on the side of the light-transparent substrate towards the non-single-crystal semiconductor laminate member, electron-hole pairs are created by the light in the I-type second non-single-crystal semiconductor layer. Holes of the electron-hole pairs thus created pass through the P-type first non-single-crystal semiconductor layer to reach the first conductive layer, and electrons flow through the N-type third non-single-crystal semiconductor layer into the second conductive layer. Therefore, photocurrent is supplied to a load which is connected between the first and second conductive layers, thus providing a photoelectric conversion function.
In conventional photoelectric conversion devices of the type described above, however, since the I-type second non-single-crystal semiconductor layer is formed to contain oxygen with a concentration above 1020 atoms/cm3, and/or carbon with a concentration above 1020 atoms/cm3, and/or phosphorus with a concentration as high as above 5xc3x971017 atoms/cm3, the I-type non-single-crystal semiconductor layer inevitably contains impurities imparting N conductivity type, with far lower concentrations than in the P-type first non-single-crystal semiconductor layer and the N-type third non-single-crystal semiconductor layer.
In addition, the impurity concentration has such a distribution that it undergoes substantially no variations in the thickness direction of the layer.
On account of this, in the case where the second non-single-crystal semiconductor layer is formed thick with a view to creating therein a large quantity of electron-hole pairs in response to the incidence of light, a depletion layer, which spreads into the second non-single-crystal semiconductor layer from the Pi junction defined between the P-type first and the I-type second non-single-crystal semiconductor layers, and a depletion layer, which spreads into the second non-single-crystal semiconductor layer from the NI junction defined between the N-type third and the I-type second non-single-crystal semiconductor layers, are not linked together. In consequence, the second non-single-crystal semiconductor layer has, over a relatively wide range thickness thereof at the central region in that direction, a region in which the bottom of the conduction band and the top of the valence band of its energy band are not inclined in the directions necessary for the holes and electrons to drift towards the first and third non-single-crystal semiconductor layers, respectively. Therefore, the holes and electrons of the electron-hole pairs created by the incident light in the second non-single-crystal semiconductor layer, in particular, the electrons and holes generated in the central region of the second layer in its thickness direction, are not effectively directed to the first and third non-single-crystal semiconductor layers, respectively.
Accordingly, the prior art photoelectric conversion devices of the above-described structure have the defect that even if the second non-single-crystal semiconductor layer is formed thick for creating a large quantity of electron-hole pairs in response to incident light, a high photoelectric conversion efficiency cannot be obtained.
Further, even if the I-type second non-single-crystal semiconductor layer is thick enough to permit the depletion layer extending into the second non-single-crystal semiconductor layer from the Pi junction between the P-type first non-single-crystal semiconductor layer on the side on which light is incident and the I-type second non-single-crystal semiconductor layer formed on the first semiconductor layer and the depletion layer extending into the second non-single-crystal semiconductor layer from the NI junction between the N-type third non-single-crystal semiconductor layer on the side opposite from the side of the incidence of light and the I-type second non-single-crystal semiconductor layer to be linked together, the expansion of the former depletion layer diminishes with the lapse of time for light irradiation by virtue of a known light irradiation effect commonly referred to as the Staebler-Wronski effect, because the I-type non-single-crystal semiconductor layer forming the PI junction contains impurities which impart the N conductivity type as mentioned previously. Finally, the above said depletion layers are disconnected from each other. In consequence, there is formed in the central region of the second nor-single-crystal semiconductor layer in the thickness direction thereof a region in which the bottom of the conduction band and the top of the valence band of the energy band are not inclined in the directions in which the holes and electrons of the electron-hole pairs created by the incidence of light are drifted towards the first and third non-single-crystal semiconductor layers, respectively.
Accordingly, the conventional photoelectric conversion devices of the above said construction have the defect that the photoelectric conversion efficiency is impaired by the long-term use of the devices.
It is therefore an object of the present invention to provide a novel photoelectric conversion device which is able to achieve a far higher photoelectric conversion efficiency than that obtainable with the conventional devices described above.
Another object of the present invention is to provide a novel photoelectric conversion device the photoelectric conversion efficiency of which is hardly or only slightly lowered by the Staebler-Wronski effect even if it is used for a long period of time.
Yet another object of the present invention is to provide a novel method which permits easy manufacture of the photoelectric conversion device having the above said excellent features.
In accordance with an aspect of the present invention, the first (or third) non-single-crystal semiconductor layer of the non-single-crystal laminate member is a layer on the side on which light is incident and is of the P conductivity type, and the I-type second non-single-crystal semiconductor layer has introduced therein an impurity for imparting thereto P-type conductivity, which is distributed so that the impurity concentration continuously decreases towards the third (or first) non-single-crystal semiconductor layer in the thickness direction of the I-type layer.
In this case, for example, the substrate is light-transparent and, accordingly, the first non-single-crystal semiconductor layer is disposed on the side where light is incident. The first and third non-single-crystal semiconductor layers are P- and N-type, respectively, and the I-type second non-single-crystal semiconductor layer has introduced therein an impurity for imparting thereto P-type conductivity, such as boron, so that its concentration in the region adjacent the first non-single-crystal semiconductor layer is higher than the concentration in the region adjacent the third non-single-crystal semiconductor layer.
On account of this, even if the I-type second non-single-crystal semiconductor layer is formed relatively thick for creating therein a large quantity of electron-hole pairs in response to the incidence of light, the depletion layer extending into the second non-single-crystal semiconductor layer from the PI junction between the first and second non-single-crystal semiconductor layers and the depletion layer extending into the second non-single-crystal layer from the NI junction between the third and second non-single-crystal semiconductor layers are joined together. Accordingly, the holes and electrons which are produced in the central region of the second non-single-crystal semiconductor layer in its thickness direction are also effectively drifted towards the first and third non-single-crystal semiconductor layers, respectively.
Moreover, even if the I-type second non-single-crystal semiconductor layer contains impurities which impart thereto N-type conductivity, because it is formed to contain oxygen and/or carbon and phosphorus in large quantities as described previously, boron, which imparts P-type conductivity and is introduced into the second non-single-crystal semiconductor layer, combines with oxygen, and/or carbon, and/or phosphorus. Besides, the P-type impurity introduced into the second non-single-crystal semiconductor layer has a high concentration in the region thereof adjacent the P-type first non-single-crystal semiconductor layer, that is, on the side of the PI junction. Therefore, the expansion of the depletion layer spreading into the second non-single-crystal semiconductor layer from the PI junction between the first and second non-single-crystal semiconductor layers is scarcely or only slightly diminished by the light irradiation effect (the Staebler-Wronski effect).
Accordingly, the photoelectric conversion device of the present invention retains a high photoelectric conversion efficiency, even if used for a long period of time.
In accordance with another aspect of the present invention, the second non-single-crystal semiconductor layer, which has introduced thereinto an impurity which imparts P-type conductivity, with such a distribution that its concentration continuously decreases towards the N-type third (or first) non-single-crystal semiconductor layer in the thickness direction of the second layer, can easily be formed, through a CVD (Chemical Vapor Deposition) method using a semiconductor material gas and an impurity material gas for imparting P-type conductivity, merely by continuously decreasing (or increasing) the concentration of the depart material gas relative to the semiconductor material gas with the lapse of time.
Accordingly, the manufacturing method of the present invention allows ease in the fabrication of the photoelectric conversion device of the present invention which possesses the aforementioned advantages.